1. Field of the Invention
The present invention relates to a display device using a light emitting element and belongs to a technical field of a large-sized display device having high resolution.
2. Description of the Related Art
Recently, a display device for displaying an image has been more and more important. At present, a liquid crystal display device that displays an image using a liquid crystal element is widely used, taking advantages of high-definition, thinness and lightness in weight. Further, a display device (a light emitting device) using a light emitting element such as organic light emitting diode (OLED) has being developed as another display device. The light emitting device using OLED (OLED display device) draws keen attention because the light emitting device has advantages such as a high response speed, superior moving image display and a wide viewing characteristic in addition to the advantages of existing liquid crystal display devices. An OLED adopted in the light emitting device as a typical light emitting element has a structure which includes a single thin film or a laminated thin film between a conductive anode and a conductive cathode. Organic materials are included in a part of or all layers of the thin film. It is usual that the luminance of the organic light emitting diode is in directly proportion to the current value thereof.
Hereinafter, a light emitting device has a light emitting element (e.g. OLED) and a plurality of pixels having at least two transistors arranged in a matrix pattern. A transistor that serially connects to a light emitting element and controls the luminance thereof in pixels is referred to as a driving transistor. A video signal of current or voltage value type is used to control pixels. When the video signal of voltage value type is used, a signal voltage is generally input to a gate electrode of a driving transistor to control the luminance of a light emitting element using the driving transistor. When the video signal of current value type is used, a light emitting device is provided with a current equivalent to a predetermined current value type from a driving transistor to control the luminance of the light emitting element. Whether the video signal is of current value type or voltage value type, there are two cases: a case where an analog value signal is used (hereinafter, referred to as an analog driving) and a case where a digital value signal is used (hereinafter, referred to as a digital driving). When the digital driving is performed, the digital driving can be combined with a time-division driving by which intermediate gray scale is displayed using a time ratio (e.g. Japanese Patent Laid-Open No. 2001-5426) or an area-division driving by which intermediate gray scale is displayed using an area ratio (e.g. Japanese Patent Laid-Open No. 2002-278478). The response speed of OLED is higher than that of a liquid crystal or the like, therefore OLED is suitable for the time-division driving in case of the digital driving.
Here are described schematically a pixel portion and a driver circuit of a display device operating conventional matrix display with reference to FIG. 7. The pixel portion is composed of a plurality of scanning lines that are arranged in the row direction of horizontal scanning, a plurality of data lines that are arranged in the column direction perpendicular to the rows and a matrix of pixels. In this manner, a plurality of pixels are regularly arranged in the pixel portion and one scanning line and one data line are also arranged in one row and one column, respectively.
When the frequency of a frame is constant, one horizontal scanning period become shorter with raising resolution of a pixel portion. For example, when the frequency of a frame is 60 Hz and the number of pixels is SXGA standard (1280×1024), one horizontal scanning period is about 16 μsec. At this time, it is difficult to obtain the period to write a video signal in a pixel. In particular, this trend is noticeable for a large-sized display whose parasitic capacitance is large.
Here are specific examples described. Firstly, a digital time-division gray scale is described, whether a video signal is of current value type or voltage value type. When one frame is divided to about 15 sub frames to perform the time-division driving, one horizontal scanning period in case that the number of pixels is SXGA standard (1280×1024) is typically 1 μsec. or less, therefore the period to write in is insufficient.
Next, an analog driving using a video signal of current value type is described here. In displaying low luminescent gray scale whose video signal current applied to an light emitting element is low, the speed to write in is sluggish and therefore the period to write in is insufficient in practical.